The present invention relates generally to apparatus for optically reading and recording high density information, digital or analog, on the surface of a record medium and, more particularly, to apparatus for optically reading and recording data at extremely high data rates, for example, where a plurality of light beams are used to record information simultaneously in multiple tracks.
High density optical disc recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Pat. No. 4,097,895 (Spong) describes an optical disc record-playback system wherein data is recorded in the form of pits in an absorbtive coating on the surface of an optical disc. In the Spong system, approximately 10.sup.11 bits of information can be stored on one side of a disc-shaped record medium having a 30 cm diameter.
Optical disc data storage systems are also known where the rate at which the information is recorded or played back is increased two, three or more times over a Spong type system by recording and playing back multiple tracks of information simultaneously. In U.S. patent application Ser. No. 288,550 of C. W. Reno, filed July 30, 1981, a multiple beam optical record and playback apparatus is described. In the Reno apparatus, a single light beam from a radiation source is split into a plurality of read and record beams. Each of the record beams which is focused to a diffraction limited spot is individually modulated by the recording signals. Data is recorded as a disturbance on the surface of the disc along a plurality of spiral tracks.
Current optical discs data storage systems for very high data rate mass memory systems include requirements for near-continuous recording of data using two separate turntables in a single machine. In such systems the data is received for a period of time that is greater than the time necessary to fill a single disc. Therefore, a first disc is filled, then a second, followed by a third, and so on until the stream of information stops. In light of this, to reduce costs and complexities in such a system, as well as power requirements, an optical system is provided that allows the record laser and modulator used for single turntable systems to be shared with a second turntable. These systems require a switch to redirect a high powered laser beam from one turntable to another with high efficiency, speed and reliability as well as beam pointing accuracy. In practice the switch device must be capable of switching a high power laser beam (approximately 1-10 watts) with an efficiency of 98% or greater. In operation, the switch should operate very rapidly, typically less than 5 milliseconds.
Prior art techniques of switching laser beams are unsatisfactory for one reason or another. For example, a galvanometer controlled mirror may be used to switch a laser beam from one position to another, however, the galvanometer has a tendency to drift out of position unless the arrangement is provided with precision stops. Further, the galvanometer controlled mirror is very sensitive to mirror angular misalignment and extremely sensitive to vibrations.
Another technique known in the prior art is to use a mirror or glass plate interposed in the beam path to redirect a laser beam. In general, mirrors or glass plates are much too slow for the high data rate systems described herein, typically it takes 10 to 20 milliseconds for the beam to switch. Further, the reliability of such a mechanical system is very low. Also, they are very sensitive to position. For example, with respect to a mirror the light reflected by the mirror is deflected by twice the angle of the mirror, thus the misalignment is twice the angular deviation of the mirror. With respect to a glass plate the glass plate would have to be very thick to provide adequate beam displacement. Since the beams lateral displacement through a glass plate is proportional to the thickness, for a given angle, the glass plate has a very high sensitivity to angle changes. Angular misalignment is a prime concern in this system because the output beams must be very stable. Another problem associated with a mirror or glass plate is that they may be temperature sensitive.
An acousto-optic modulator is not suitable for a laser beam switch. The optical efficiency of such a system is very low, on the order of 70%. Another problem is that the deflection angle of such a system is very small, typically on the order of 10-30 milli-radians. Further, an acousto-optic modulator is fairly complex because it requires a long throw or the use of other optics to allow pick off of the diffracted beam. Also, the focused spot may create energy densities that would destroy anti-reflection coatings on optical elements. One other drawback of the acousto-optic modulator is that the drive electronics are generally complicated.
A fourth technique used in the prior art for beam switching is the use of an electro-optic modulator. Electro-optic modulators are very expensive and very bulky. Further, they are extremely temperature sensitive. To avoid the bulkiness of such a modulator a very large half-wave voltage must be used. But in this regard, high voltages are being traded for large crystals.